Methods and apparatuses for assisted navigation systems

ABSTRACT

The invention relates to a navigation systems and elements. A network element (M) includes a receiver (M. 2.2 ) for forming assistance data relating to at least one navigation system. The network element (M) inserts indication of the navigation system and a selected mode into the assistance data and constructs the assistance data according to the selected mode. The network element (M) has a transmitting element (M. 3.1 ) for transmitting the assistance data via a communications network (P) to a device (R). The device (R) includes a positioning receiver (R. 3 ) for performing positioning on the basis of one or more signals of the at least one satellite navigation system; a receiver (R. 2.2 ) for receiving the assistance data from the network element (M); and an examining element (R. 1.1 ) adapted to examine the received assistance data. The assistance data is adapted to be used by the positioning receiver for performing positioning of the device (R).

This application is the National Stage of International Application No.PCT/FI2006/050084, International Filing Date, 28 Feb. 2006, whichdesignated the United States of America.

FIELD OF THE INVENTION

This invention relates to a field of assisted navigation systems andmore specifically to a format, in which assistance data is distributedfrom a communications network to terminals. The invention also relatesto a device comprising a positioning receiver for performing positioningon the basis of one or more signals of a satellite navigation system.The invention also relates to a network element comprising a transmitterfor transmitting assistance data of a satellite navigation system to areceiver. The invention further relates to a method, a computer programproduct and a signal for delivering assistance data of a satellitenavigation system to a positioning receiver.

BACKGROUND OF THE INVENTION

One known navigation system is the GPS system (Global PositioningSystem) which presently comprises more than 24 satellites, of whichusually a half of them are simultaneously within the sight of areceiver. These satellites transmit e.g. Ephemeris data of thesatellite, as well as data on the time of the satellite. A receiver usedin positioning normally deduces its position by calculating thepropagation time of signals received simultaneously from severalsatellites belonging to the positioning system to the receiver andcalculates the time of transmission (ToT) of the signals. For thepositioning, the receiver must typically receive the signal of at leastfour satellites within sight to compute the position. The other alreadylaunched navigation system is the Russian-based GLONASS (Global'nayaNavigatsionnaya Sputnikovaya Sistema).

In the future, there will also exist other satellite based navigationsystems than GPS and GLONASS. In the Europe the Galileo system is underconstruction and will be operational within a few years. Space BasedAugmentation Systems SBAS (Wide Area Augmentation System WMS, EuropeanGeostationary Navigation Overlay Service EGNOS, GPS Aided GEO AugmentedNavigation GAGAN) are also being ramped up. Local Area AugmentationSystems LAAS, which uses fixed navigation stations on the ground, arebecoming more common. Strictly speaking, the Local Area AugmentationSystems are not actually satellite based navigation systems although thenavigation stations are called as “pseudo satellites” or “pseudolites”.The navigation principles applicable with the satellite based systemsare also applicable with the Local Area Augmentation Systems. Pseudolitesignals can be received with a standard GNSS (Global NavigationSatellite System) receiver. Moreover, Japanese are developing their ownGPS/Galileo complementing system called Quasi-Zenith Satellite SystemQZSS.

The satellite based navigation systems, including systems using pseudosatellites, can collectively be called as Global Navigation SatelliteSystems (GNSS). In the future there will probably be positioningreceivers which can perform positioning operations using, eithersimultaneously or alternatively, more than one navigation system. Suchhybrid receivers can switch from a first system to a second system ife.g. signal strengths of the first system fall below a certain limit, orif there are not enough visible satellites of the first system, or ifthe constellation of the visible satellites of the first system is notappropriate for positioning. Simultaneous use of different systems comesinto question in challenging conditions, such as urban areas, wherethere is limited number of satellites in view. In such cases, navigationbased on only one system is practically impossible due to the lowavailability of signals. However, hybrid use of different navigationsystems enables navigation in these difficult signal conditions.

Each satellite of the GPS system transmits a ranging signal at a carrierfrequency of 1575.42 MHz called L1. This frequency is also indicatedwith 154f₀, where f₀=10.23 MHz. Furthermore, the satellites transmitanother ranging signal at a carrier frequency of 1227.6 MHz called L2,i.e. 120f₀. In the satellite, the modulation of these signals isperformed with at least one pseudo random sequence. This pseudo randomsequence is different for each satellite. As a result of the modulation,a code-modulated wideband signal is generated. The modulation techniqueused makes it possible in the receiver to distinguish between thesignals transmitted from different satellites, although the carrierfrequencies used in the transmission are substantially the same. Dopplereffect results in a small (±1 kHz) change in the carrier frequencydepending upon the constellation geometry. This modulation technique iscalled code division multiple access (CDMA). In each satellite, formodulating the L1 signal, the pseudo sequence used is e.g. a so-calledC/A code (Coarse/Acquisition code), which is a code from the family ofthe Gold codes. Each GPS satellite transmits a signal by using anindividual C/A code. The codes are formed as a modulo-2 sum of two1023-bit binary sequences. The first binary sequence G1 is formed with apolynomial X¹⁰+X³+1, and the second binary sequence G2 is formed bydelaying the polynomial X¹⁰+X⁹+X⁸+X⁶+X³+X²+1 in such a way that thedelay is different for each satellite. This arrangement makes itpossible to produce different C/A codes with an identical codegenerator. The C/A codes are thus binary codes whose chipping rate inthe GPS system is 1.023 MHz. The C/A code comprises 1023 chips, whereinthe code epoch is 1 ms. The L1 carrier signal is further modulated withnavigation information at a bit rate of 50 bit/s. The navigationinformation comprises information about the health of the satellite, itsorbit, time data, etc.

In the GPS system, satellites transmit navigation messages includingEphemeris data and time data, which are used in the positioning receiverto determine the position of the satellite at a given instant. TheseEphemeris data and time data are transmitted in frames which are furtherdivided into subframes. FIG. 6 shows an example of such a framestructure FR. In the GPS system, each frame comprises 1500 bits whichare divided into five subframes of 300 bits each. Since the transmissionof one bit takes 20 ms, the transmission of each subframe thus takes 6s, and the whole frame is transmitted in 30 seconds. The subframes arenumbered from 1 to 5. In each subframe 1, e.g. time data is transmitted,indicating the moment of transmission of the subframe as well asinformation about the deviation of the satellite clock with respect tothe time in the GPS system.

The subframes 2 and 3 are used for the transmission of Ephemeris data.The subframe 4 contains other system information, such as universaltime, coordinated (UTC). The subframe 5 is intended for the transmissionof almanac data on all the satellites. The entity of these subframes andframes is called a GPS navigation message which comprises 25 frames, or125 subframes. The length of the navigation message is thus 12 min 30 s.

In the GPS system, time is measured in seconds from the beginning of aweek. In the GPS system, the moment of beginning of a week is midnightbetween a Saturday and a Sunday. Each subframe to be transmittedcontains information on the moment of the GPS week when the subframe wastransmitted. Thus, the time data indicates the moment of transmission ofa certain bit, i.e. in the GPS system, the moment of transmission of thelast bit in the subframe. In the satellites, time is measured withhigh-precision atomic chronometers. In spite of this, the operation ofeach satellite is controlled in a control centre for the GPS system (notshown), and e.g. a time comparison is performed to detect chronometricerrors in the satellites and to transmit this information to thesatellite.

The number of satellites, the orbital parameters of the satellites, thestructure of the navigation messages, etc. may be different in differentnavigation systems. Therefore, the operating parameters of a GPS basedpositioning receiver may not be applicable in a positioning receiver ofanother satellite system. On the other hand, at least the designprinciples of the Galileo has indicated that there will be somesimilarities between GPS and Galileo in such a way that at least Galileoreceiver should be able to utilize GPS satellite signals in positioning.

Positioning devices (or positioning receivers) i.e. devices which havethe ability to perform positioning on the basis of signals transmittedin a navigation system can not always receive strong enough signals fromthe required number of satellites. For example, it may occur that when athree-dimensional positioning should be performed by the device, it cannot receive signals from four satellites. This may happen indoors, inurban environments, etc. Methods and systems have been developed forcommunications networks to enable position in adverse signal conditions.If the communications network only provides navigation model assistanceto the receiver, the requirement for a minimum of three signals intwo-dimensional positioning, or four signals in three-dimensionalpositioning does not diminish. However, if the network provides, forinstance, barometric assistance, which can be used for altitudedetermination, then three satellites is enough for three-dimensionalpositioning. These so called assisted navigation systems utilise othercommunication systems to transmit information relating to satellites tothe positioning devices. Respectively, such positioning devices whichhave the ability to receive and utilize the assistance data can becalled as assisted GNSS receivers, or more generally, assistedpositioning devices.

Currently, only assistance data relating to GPS satellites can beprovided to assisted GNSS receivers in CDMA (Code Division MultipleAccess), GSM (Global System for Mobile communications) and W-CDMA(Wideband Code Division Multiple Access) networks. This assistance dataformat closely follows the GPS navigation model specified in theGPS-ICD-200 SIS (ICD, Interface Control Document; SIS, Signal-In-Space)specification. This navigation model includes a clock model and an orbitmodel. To be more precise, the clock model is used to relate thesatellite time to the system time, in this case the GPS time. The orbitmodel is used to calculate the satellite position at a given instant.Both data are essential in satellite navigation.

The availability of the assistance data can greatly affect thepositioning receiver performance. In the GPS system, it takes at least18 seconds (the length of the first three subframes) in good signalconditions for a GPS receiver to extract a copy of the navigationmessage from the signal broadcasted by a GPS satellite. Therefore, if novalid copy (e.g. from a previous session) of a navigation model isavailable, it takes at least 18 seconds before the GPS satellite can beused in position calculation. Now, in AGPS receivers (Assisted GPS) acellular network such as GSM or UMTS (Universal MobileTelecommunications System) sends to the receiver a copy of thenavigation message and, hence, the receiver does not need to extract thedata from the satellite broadcast, but can obtain it directly from thecellular network. The time to first fix (TTFF) can be reduced to lessthan 18 seconds. This reduction in the time to first fix may be crucialin, for instance, when positioning an emergency call. This also improvesuser experience in various use cases, for example when the user isrequesting information of services available nearby the user's currentlocation. These kinds of Location Based Services (LBS) utilize in therequest the determined location of the user. Therefore, delays in thedetermination of the location can delay the response(s) from the LBS tothe user.

Moreover, in adverse signal conditions the utilization of the assisteddata may be the only option for navigation. This is because a drop inthe signal power level may make it impossible for the GNSS receiver toobtain a copy of the navigation message. However, when the navigationdata is provided to the receiver from an external source (such as acellular network), navigation is enabled again. This feature can beimportant in indoor conditions as well as in urban areas, where signallevels may significantly vary due to buildings and other obstacles,which attenuate satellite signals.

The international patent application publication WO 02/67462 disclosesGPS assistance data messages in cellular communications networks andmethods for transmitting GPS assistance data in cellular networks.

When a mobile terminal having an assisted positioning receiver requestsfor assistance data, the network sends the mobile terminal onenavigation model for each satellite in the view of the assistedpositioning receiver. The format in which the assistance data is sent isspecified in various standards. Control Plane solutions include RRLP(Radio Resource Location Services Protocol) in GSM, RRC (Radio ResourceControl) in W-CDMA and IS-801.1/IS-801.A in CDMA. Broadcast assistancedata information elements are defined in the standard TS 44.035 for GSM.Finally, there are User Plane solutions OMA SUPL 1.0 (Open MobileAlliance, Secure User Plane for Location) and various proprietarysolutions for CDMA networks. The common factor for all these solutionsis that they only support GPS.

However, due to the ramp up of Galileo, all the standards shall bemodified in the near future in order to achieve Galileo compatibility.

Hence, it is clear that GPS assistance alone will not be adequate in thenear future and a new data format must be developed in order to be ableto support the new systems.

The problem in providing assistance data for new systems, as well as toGPS, can be reduced to finding a navigation model (clock and orbitmodel) that can be used to describe all the satellite systems. Astraightforward solution is to take the native navigation message formatfor each of the systems and use this format. However, this would resultin various different messages (different message format for each system)which would make the implementation task problematic. Moreover, thenative format may also be incompatible with cellular standards.Therefore, the final solution must be such that various differentformats are not required.

The challenges in developing a common format include firstly satelliteindexing. The satellite index is used to identify the navigation modelwith a specific satellite. The problem is that every system has its ownindexing method.

GPS indexes satellites (SV, Space Vehicle) based on PRN (Pseudo-RandomNoise) numbers. The PRN number can be identified with the CDMA spreadcode used by the satellites.

Galileo uses a 7-bit field (1-128) to identify the satellite. The numbercan be identified with the PRN code used by the satellite.

GLONASS uses a 5-bit field to characterize satellites. The number can beidentified with the satellite position in the orbital planes (thisposition is called a “slot”). Moreover, in contrast to other systems,GLONASS uses FDMA (Frequency Division Multiple Access) to spreadsatellite broadcasts in spectrum. It is noted here that there is also aCDMA spread code in use in the GLONASS. There is, therefore, a tablethat maps the satellite slot number to the broadcast frequency. This mapmust be included in any assistance data format.

SBAS systems use PRN numbers similar to GPS, but they have an offset of120. Therefore, the first satellite of the SBAS system has a satellitenumber of 120.

Since QZSS SIS ICD is not public yet, there is no detailed informationon the satellite indexing in the system. However, since the system is aGPS augmentation, the GPS compatible format should at high probabilitybe compatible with QZSS as well.

Pseudolites (LAAS, Local Area Augmentation System) are the mostproblematic in the indexing sense. There is no standard defined forindexing pseudolites currently. However, the indexing should at leastloosely follow the GPS-type indexing, since they use GPS-type PRNs.Therefore, by ensuring that the range of satellite indices issufficient, it should be possible to describe LAAS transmitters withGPS-type satellite indexing.

The second challenge is the clock model. The clock model for any systemis given byt _(SYSTEM)(t)=t _(SV)(t)−[a ₀ +a ₁·(t _(SYSTEM)(t)−t _(REFERENCE))+a₂·(t _(SYSTEM)(t)−t _(REFERENCE))²]where t_(SYSTEM)(t) is the system time (for instance, GPS time) atinstant t, t_(SV)(t) satellite time at instant t, t_(REFERENCE) is themodel reference time and a_(i) (iε{0,1,2}) are the 0^(th), 1^(st) and2^(nd) order model coefficients, respectively. Relativistic correctionterm is not shown in the equation. Since the equation is the same foreach system, the only problem in developing the generalized model is tofind such bit counts and scale factors that the ¹⁾range of valuesrequired by each system is covered and the ²⁾accuracy (or resolution)requirements for each system are met.

The third problem includes the orbit model. Again, each system has itsown format (excluding GPS and Galileo that use the same format). GPS andGalileo use Keplerian orbit parameter set: 6 orbit parameters, 3 linearcorrection terms as well as 6 harmonic gravitation correction terms. Incontrast to GPS and Galileo, GLONASS navigation model only containsinformation on the satellite position, velocity and acceleration at agiven instant. This information can then be used (by solving an initialvalue problem for the equations of motion) to predict the satelliteposition at some instant. SBAS utilizes in some sense format similar toGLONASS. The SBAS navigation message includes information on thesatellite position, velocity and acceleration in ECEF (Earth-CenteredEarth Fixed coordinate system definition) systems at a given instant.This data is used to predict the satellite position by simpleextrapolation, which is in contrast with GLONASS, in which equations ofmotion are integrated in time. Again, since the QZSS ICD is notavailable yet, the detailed format of the navigation message is notknown. However, there are documents that quote the QZSS signal beingcompatible with either GPS-type ephemeris or SBAS-type broadcast. Hence,ensuring that the new format is compatible with GPS and SBAS, the QZSSorbits may be described using the format of GPS. LAAS require that theorbit model is able to describe objects that are stationary in theECEF-frame. Also, pseudolites have fairly strict resolution requirementsfor the position. It is necessary to be able to describe a pseudoliteposition at a resolution of about 5 mm in some cases.

In addition to these requirements (indexing, clock model and orbitmodel), the navigation model must include information on model referencetime (t_(REFERENCE) in the clock model, similar time stamp is requiredfor the orbit model), model validity period, issue of data (in order tobe able differentiate between model data sets), and SV health (indicateswhether navigation data from the SV is usable or not).

Needless to say, almost all the systems have their own method ofexpressing these items. The range and accuracy requirements vary fromsystem to system. Moreover, the current satellite health field requiresmodification, since in the future GPS (and other systems) do nottransmit only one signal, but various signals at different frequencies.

Now, the new assistance data format must be such that all the systemspecific items as well as parameter range and accuracy requirements aretaken into account.

Finally, the problem with current assistance data format is that it onlyallows for one set of navigation data to be available for a givensatellite. This means that when the navigation model is updated, theterminal must be provided with a new set of data. However, already nowthere are commercial services that provide navigation data that is validfor 5-10 days. The navigation model validity time does not increase, butthe service sends multiple sets of navigation data for one satellite. Inassisted GNSS this is advantageous, since the user receives all theassistance needed for the next couple of weeks in a single download. Thenew assistance data format must, therefore, be able to support theselong-term orbit fits in current models.

To this date there has been no solution to the problem. This is becausethe assistance data distribution has been limited to GPS-system and toCDMA-networks.

The current solution in distributing assistance data to the terminals isto obtain navigation model for GPS directly from the satellitebroadcasts, modify these data and distribute it to terminals in thenetwork according to various standards in use.

SUMMARY OF THE INVENTION

The current invention includes a generalized navigation model, which canbe used to characterize the satellite clock behaviour and the satelliteorbit in more than one navigation system. The generalized navigationmodel can be used at least with GPS, Galileo, GLONASS, SBAS, LAAS andQZSS. There are also reservations for yet unknown future systems.

The indexing problem has been solved by extending the satellite indexfield in such a way that the upper bits of the field define thenavigation system (GPS, Galileo, GLONASS, SBAS, LAAS, QZSS or somefuture system) and the lower bits express the satellite index in thesystem native format. The field shall be called SS index from now on todenote “System and Satellite”. There is also a GLONASS specific additionthat allows for mapping the SS index to the satellite broadcastfrequency (or channel).

The clock model problem has been solved by finding such bit counts andscale factors for the coefficients that the clock models in all thesystems can be described by using the generalized clock model. However,the invention does not exclude using different clock models for eachsystem.

The orbit model problem has been solved by introducing a multi-modemodel. The modes of the model are, for instance, Mode 1: Keplerianmodel; Mode 2: Position in ECEF-coordinates; and Mode 3: Position,velocity and acceleration in ECEF-coordinates. More modes may be added,if such a need arises. An example embodiment, of the idea is that theupper bits of the SS index (i.e. system) define the model mode. However,also other implementations can be used to indicate the model mode, forexample by using a mode index. The modes are mutually exclusive.

Long-term orbit fits do not require anything special. The reference timeand validity period define precisely, when the model can be used. Iflong-term data is available, the network provides the terminal with thelong-term data and it is the responsibility of the terminal to take careof storing and handling multiple sets of navigation data for the samesatellite (or SS index). However, if the navigation model is not basedon the broadcasted navigation model, but is long-term data, this couldbe indicated, for example, in the Issue-of-Data field, but also otherimplementations are possible.

According to a first aspect of the present invention there is provided adevice comprising

-   -   a positioning receiver for performing positioning on the basis        of one or more signals of at least one satellite navigation        system;    -   a receiver for receiving assistance data relating to at least        one navigation system; and    -   an examining element adapted to examine the received assistance        data;        characterised in that the device further comprises    -   a determining element adapted to determine the mode of the        assistance data in said assistance data, wherein said assistance        data is adapted to be used by the positioning receiver for        performing positioning of the device.

According to a second aspect of the present invention there is provideda network element comprising

-   -   a controlling element for forming assistance relating to at        least one navigation system; and    -   a transmitting element for transmitting assistance data to a        communications network;        characterised in that the controlling element is adapted to    -   select a mode for the transmission of the assistance data;    -   insert indication of the navigation system and the selected mode        into the assistance data; and    -   construct the assistance data according to the selected mode.

According to a third aspect of the present invention there is provided asystem comprising:

-   -   network element which comprises        -   a controlling element for forming assistance data relating            to at least one navigation system; and        -   a transmitting element for transmitting the assistance data            to a communications network;    -   a device which comprises        -   a positioning receiver for performing positioning on the            basis of one or more signals of said at least one satellite            navigation system;        -   a receiver for receiving assistance data from the network            element; and        -   an examining element adapted to examine the received            assistance data;            characterised in that the controlling element is adapted to    -   select a mode for the transmission of the assistance data;    -   insert indication of the navigation system and the selected mode        into the assistance data; and    -   construct the assistance data according to the navigation        system;        and that the device further comprises    -   a determining element adapted to determine the mode of the        assistance data in said assistance data, wherein said assistance        data is adapted to be used by the positioning receiver for        performing positioning of the device.

According to a fourth aspect of the present invention there is provideda module for a device comprising a positioning receiver for performingpositioning on the basis of one or more signals of at least onesatellite navigation system; said module comprising

-   -   a receiving element for receiving assistance data relating to at        least one navigation system;    -   an examining element adapted to examine the received assistance        data;        characterised in that the module further comprises    -   a determining element adapted to determine the mode of the        assistance data in said assistance data, and    -   an output to transfer indication on the mode of the assistance        data to the positioning receiver,    -   wherein said assistance data is adapted to be used by the        positioning receiver for performing positioning of the device.

According to a fifth aspect of the present invention there is provided amethod for transmitting assistance data to a device, the methodcomprising:

-   -   forming assistance data relating to at least one navigation        system; and    -   transferring assistance data to the device;        characterised in that the method further comprises    -   determining the navigation system the navigation data relates        to;    -   selecting a mode for transmitting the assistance data;    -   inserting an indication of the navigation system and the        selected mode into the assistance data; and    -   constructing the assistance data according to the selected mode.

According to a sixth aspect of the present invention there is provided acomputer program product for storing computer program having computerexecutable instructions for

-   -   forming assistance data relating to at least one navigation        system; and    -   transmitting assistance data to a device;        characterised in that the computer program further comprises        computer executable instructions for    -   determining the system the navigation data relates to;    -   selecting a mode for transmitting the assistance data;    -   inserting indication of the navigation system and the selected        mode into the assistance data; and    -   constructing the assistance data according to the selected mode.

According to a seventh aspect of the present invention there is provideda signal for delivering assistance data to a device, the signalcomprising

-   -   assistance data relating to at least one navigation system;        characterised in that the signal further comprises    -   an indication of the navigation system the assistance data        relates to and a mode selected for transmitting the assistance        data;        wherein said assistance data has been constructed according to        the selected mode.

According to an eighth aspect of the present invention there is provideda carrier having a signal recorded thereon for delivering assistancedata to a device, the signal comprising

-   -   assistance data relating to at least one satellite navigation        system;        characterised in that the signal further comprises    -   an indication of the navigation system the assistance data        relates to and a mode selected for transmitting the assistance        data;        wherein said assistance data has been constructed according to        the selected mode.

According to a ninth aspect of the present invention there is providedan assistance data server comprising

-   -   a controlling element for forming assistance relating to at        least one navigation system; and    -   a transmitting element for transmitting assistance data to a        communications network;        characterised in that the controlling element is adapted to    -   select a mode for the transmission of the assistance data;    -   insert indication of the navigation system and the selected mode        into the assistance data; and    -   construct the assistance data according to the selected mode.

The invention shows some advantages over prior art. The format accordingto the invention is suitable for a number of cellular standards and fora number of GNSS systems. These characteristics make the currentinvention a very attractive solution since a globally applicablesolution reduces implementation costs. This applies to handsetmanufacturers as well as to operators of communications networks andpossible commercial assistance data service providers. The prior artimplementation in RRLP and RRC only include the possibility to providethe Assisted GPS receiver with GPS assistance data. There has been nopossibility to support Galileo, GLONASS, SBAS, LAAS or QZSS. This hasbeen a drawback and can be corrected for using the present invention.Since Galileo assistance data will almost certainly be included in RRLPand RRC, there is now possibility to make this format as general aspossible in order to able to support future systems also.

DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detail withreference to the appended drawings, in which

FIG. 1 depicts as a general, simplified diagram a system in which thepresent invention can be applied,

FIG. 2 depicts a reference receiver of a navigation system according toan example embodiment of the present invention as a simplified blockdiagram,

FIG. 3 depicts a network element according to an example embodiment ofthe present invention as a simplified block diagram,

FIG. 4 depicts a device according to an example embodiment of thepresent invention as a simplified block diagram,

FIG. 5 depicts according to an example embodiment of the presentinvention, and

FIG. 6 shows an example of a frame structure used in the GPS system.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 there is depicted an example of a system 1 which can be usedfor positioning a device R. The system 1 comprises reference stations S,such as satellites S1 of a first navigation system, for example the GPS,and satellites S2 of a second navigation system, for example theGLONASS. It should be noted here that GPS and GLONASS are only mentionedas non-limiting examples here and also other reference stations S thansatellites can be used (e.g. pseudolites of the LAAS). Also the numberof reference stations is greater than shown in FIG. 1. The navigationsystems comprise one or more ground stations G. The ground station Gcontrols the operation of the satellites S1, S2 of the navigationsystems 2, 3, respectively. The ground station G can e.g. determinedeviations of the orbits of the satellites and the accuracy of theclock(s) of the satellites (not shown). If the ground station G detectsa need to correct the orbit or the clock of a satellite S1, S2, ittransmits a control signal (or control signals) to the satellite S1, S2which then performs a correction operation on the basis of the controlsignal(s). In other words, the ground station G refer to the GroundSegment of the navigation system.

During their operation, the satellites S1, S2 monitor the condition oftheir equipment. The satellites S1, S2 may use, for example, watchdogoperations to detect and report possible faults in the equipment. Theerrors and malfunctions can be instantaneous or longer lasting. On thebasis of the health data, some of the faults can possibly be compensatedfor, or the information transmitted by a malfunctioning satellite can betotally disregarded. The malfunctioning satellite S1, S2 sets a flag ina satellite health field of a navigation message indicative of a failureof the satellite. The satellite S1, S2 can also indicate in thenavigation message a signal or signals which is/are not operatingproperly. It is also possible that the ground station G can detect thatsome satellite is not operating properly and set an indication of themalfunctioning signal(s) of that satellite. This indication can then betransmitted to the communications network P in a navigation message.

In this non-limiting example embodiment the communications network P isthe GSM network and the network element M communicating with thereference receiver C, C″ is the Mobile Switching Centre (MSC) of the GSMnetwork. The reference receiver C can transmit assistance data to thenetwork element M. The network element stores the assistance data to amemory M.4 (FIG. 3) for transmission to a device R when the device Rneeds the assistance data to perform assisted positioning operation. Itis also possible to transmit the assistance data from the networkelement M to the device R before it is needed. For example, the device Rcan request the assistance data of all visible satellites and store thenavigation data to the memory R.4 of the device R for later use.

The network element M can also be the Serving Mobile Location Centre(SMLC) of the GSM network. The Serving Mobile Location Centre is eithera separate network element (such as the MSC) or integrated functionalityin a base station B (BSC, Base Station Controller) that contains thefunctionality required to support location based services. The SMLCmanages the overall co-ordination and scheduling of resources requiredfor locating a device R. It also calculates the final location estimateand estimates the achieved accuracy. The SMLC may control a number ofLocation Measurement Units (LMU) for the purpose of obtaining radiointerface measurements to locate or help locate the mobile stationsubscribers in the area that it serves.

Now, the main elements of an example embodiment of the referencereceiver C will be described in more detail with reference to FIG. 2.The disclosure is applicable to both the reference receiver C of thefirst navigation system and the reference receiver C″ of the secondnavigation system, although practical implementations may be differentfrom each other. The reference receiver C comprises a controller C.1 forcontrolling the operation of the reference receiver C. The controllerC.1 comprises e.g. a processor, a microprocessor, a digital signalprocessor (DSP) or a combination of these. It is obvious that there canalso be more than one processor, microprocessor, DSP, etc. in thecontroller C.1. There is also a reception block C.2 comprising areceiver C.2.2 for receiving signals from the satellites S1, S2 of thenavigation system. The reference receiver C further comprises acommunication block C.3 for communicating, either directly orindirectly, with the network element M of the communications network P.The communication block C.3 comprises a transmitter C.3.1 fortransmitting signals to the network element M and, if necessary, areceiver C.3.2 for receiving signals transmitted by the network elementM to the reference receiver C. The reference receiver C may alsocomprise memory C.4 for storing data and software (computer programcode).

The structure of an example embodiment of the network element M isdepicted in FIG. 3. The network element M comprises a controller M.1.Also the controller M.1 of the network element may be constructed of aprocessor, a microprocessor, a digital signal processor (DSP) or acombination of these. It is obvious that there can also be more than oneprocessor, microprocessor, DSP, etc. in the controller M.1. The networkelement M can communicate with the reference receiver C by the firstcommunication block M.2. The first communication block M.2 comprises areceiver M.2.2 for receiving signals from the reference receivers C ofthe navigation systems. The first communication block M.2 may alsocomprise a transmitter M.2.1 for transmitting e.g. request messages tothe reference receiver C of the navigation system. The network element Mfurther comprises a second communication block M.3 for communicatingwith the base stations B or other access points of the communicationsnetwork P. The second communication block M.3 comprises a transmitterM.3.1 for transmitting signals to the base stations B and a receiverM.3.2 for receiving signals transmitted by the base stations B to thenetwork element M. The network element M also comprises memory M.4 forstoring data and software (computer program code).

The network element M obtains the assistance data either from satellitebroadcasts by using a reference receiver C or some other externalsolution, e.g. from an assistance data server X intended to gather andtransmit such information to communications networks. The assistancedata server X comprises analogous elements with the network element Mwith respect to the operations relating to the receiving navigationdata, forming and transmitting the assistance data (i.e. the receiverM.2.2, the controller M.1, the transmitter M.3.1, the memory M.4). Theassistance data server X may also comprise elements of the referencereceiver C. The assistance data server X is, for example, a server of acommercial service provider from who assistance data can be requested,maybe against a fee.

FIG. 4 depicts a device R according to an example embodiment of thepresent invention as a simplified block diagram. The device R comprisesone or more positioning receivers R.3 for receiving signals from thereference stations S1, S2 of one or more navigation systems. There canbe one positioning receiver R.3 for each navigation system the device Ris intended to support, or it may be possible to use one positioningreceiver R.3 for performing positioning on the basis of signals of morethan one navigation system. The device R also comprises a controller R.1for controlling the operation of the device R. Again, the controller R.1of the network element may be constructed of a processor, amicroprocessor, a digital signal processor (DSP) or a combination ofthese. It is obvious that there can also be more than one processor,microprocessor, DSP, etc. It is also possible that the positioningreceiver R.3 can comprise a controlling element R.3.1 (e.g. a processor,a microprocessor and/or a DSP), or the positioning receiver R.3 uses thecontroller of the device R in positioning. It is also possible that someof the positioning operations are carried out by the controlling elementR.3.1 of the positioning receiver R.3 and some other positioningoperations are carried out by the controller R.1 of the device. Thedevice R can communicate with a base station B of the communicationsnetwork P by the communication block R.2. The communication block R.2comprises a receiver R.2.2 for receiving signals from the base station Bof the communications network P. The communication block M.2 alsocomprises a transmitter R.2.1 for transmitting messages to the basestation B of the communications network P. Data and software can bestored to the memory R.4 of the device. The device R is also providedwith a user interface R.5 (UI) which comprises, for example, a displayR.5.1, a keypad R.5.2 (and/or a keyboard), and audio means R.5.3, suchas a microphone and a loudspeaker. It is also possible that the devicehas more than one user interface.

The device R is, for example, a mobile communication device intended tocommunicate with the communications network P as is known as such. Theuser interface R.5 can be common to both the mobile communication partand the positioning receiver R.3.

In the following, a non-limiting example of the assistance data formatfields are disclosed with reference to the Table 1 and FIG. 5. The orbitmodel problem has been solved by introducing a multi-mode model. Themodes of the model are at least Mode 1: Keplerian model, which supportsat least the GPS, Galileo, and QZSS systems; Mode 2: Position inECEF-coordinates, which supports at least the LAAS system; and Mode 3:Position, velocity and acceleration in ECEF-coordinates, which supportsat least the GLONASS, SBAS, and QZSS systems. There can also be morethan three modes for future systems and different kind ofimplementations of the invention.

In the Table 1, associated bit counts, scale factors and different modesare shown. The explanations follow the Table.

TABLE 1 Scale Parameter # Bits Factor Units Incl. (t_(oe)_MSB) 12 2²⁰sec Satellite and Format Identification (once per model) SS ID  9^((u))— — M Carrier Frequency  5 — — C index (Fit Interval)  6 — h C (SVHealth)  8^((u)) — Boolean C (IOD) 11^((u)) — — C Satellite Clock Model(once per model) (t_(oc)) 20^((u)) 1 sec C af₂ 18 2⁻⁶⁵ sec/sec² C af₁ 162⁻⁴³ sec/sec C af₀ 28 2⁻³³ sec C (T_(GD))  8 2⁻³¹ sec C SatelliteNavigation Model Using Keplerian Parameters (once per model) (t_(oe))20^((u)) 1 sec C⁽¹⁾, MODE 1 ω 32 2⁻³¹ semi-circles C⁽¹⁾, MODE 1 Δn 162⁻⁴³ semi- C⁽¹⁾, MODE 1 circles/sec M₀ 32 2⁻³¹ semi-circles C⁽¹⁾, MODE 1{dot over (Ω)} 24 2⁻⁴³ semi- C⁽¹⁾, MODE 1 circles/sec e 32^((u)) 2⁻³³ —C⁽¹⁾, MODE 1 {dot over (i)} 14 2⁻⁴³ semi- C⁽¹⁾, MODE 1 circles/sec(A)^(1/2) 32^((u)) 2⁻¹⁹ meters^(1/2) C⁽¹⁾, MODE 1 i₀ 32 2⁻³¹semi-circles C⁽¹⁾, MODE 1 Ω₀ 32 2⁻³¹ semi-circles C⁽¹⁾, MODE 1 C_(rs) 162⁻⁵ meters C⁽¹⁾, MODE 1 C_(is) 16 2⁻²⁹ radians C⁽¹⁾, MODE 1 C_(us) 162⁻²⁹ radians C⁽¹⁾, MODE 1 C_(rc) 16 2⁻⁵ meters C⁽¹⁾, MODE 1 C_(ic) 162⁻²⁹ radians C⁽¹⁾, MODE 1 C_(uc) 16 2⁻²⁹ radians C⁽¹⁾, MODE 1 SatelliteNavigation Model Using ECEF Coordinates (once per model) (t_(oe))20^((u)) 1 sec C⁽²⁾, MODE 2 & 3 X MSB 27 1 meters C⁽²⁾, MODE 2 & 3 Y MSB27 1 meters C⁽²⁾, MODE 2 & 3 Z MSB 27 1 meters C⁽²⁾, MODE 2 & 3 X LSB 8^((u)) 2⁻⁸ meters O⁽²⁾, MODE 2 & 3 Y LSB  8^((u)) 2⁻⁸ meters O⁽²⁾,MODE 2 & 3 Z LSB  8^((u)) 2⁻⁸ meters O⁽²⁾, MODE 2 & 3 X′ 26 2⁻¹²meters/sec O⁽²⁾, MODE 3 Y′ 26 2⁻¹² meters/sec O⁽²⁾, MODE 3 Z′ 26 2⁻¹²meters/sec O⁽²⁾, MODE 3 X″ 19 2⁻²² meters/sec² O⁽²⁾, MODE 3 Y″ 19 2⁻²²meters/sec² O⁽²⁾, MODE 3 Z″ 19 2⁻²² meters/sec² O⁽²⁾, MODE 3 SatellitePosition Accuracy Model (once per model) (r₀)  5^((u)) — meters C (r₁) 5^((u)) 2⁻¹⁸ meters/sec O NOTE 1: All of these fields shall be presenttogether, or none of them shall be present. NOTE 2: All of these fieldsshall be present only if position information for a specific satelliteis given in ECEF frame, or none of them shall be present if positioninformation for a specific satellite is given in Keplerian parameters.NOTE u: unsigned parameter

The Table 1 discloses examples of the fields and different modes 1, 2and 3. The information of Table 1 can be divided into six sections. Thefirst section contains one field t_(oe) _(—) MSB, which specifies the 12most significant bits (MSBS) of the time of ephemeris t_(oe) and thereference time for the clock model t_(oc) given in UTC Time (UniversalTime Coordinated). The device R should compensate for the possiblerollover in the time of ephemeris t_(oe) and the reference time for theclock model upon reception of the navigation model. The time ofephemeris t_(oe) and the reference time for the clock model t_(oc) havea time span of about 1.7 weeks.

The second section relates to satellite and format identification. Thesecond section exists once each mode in an assistance message A (FIG.5). The first field of the second section contains the System andSatellite Identification SS_ID. The System and Satellite Identificationis used to define different satellites and satellite systems. The Systemand Satellite Identification SS_ID is, in this non-limiting example, a9-bit field divided to 2 subfields. The first subfield (System ID)contains the ID number of the satellite system, and the second subfield(SV/Slot ID) contains the index of the satellite in the system for whichthe navigation data follows. The bit masks for the System and SatelliteIdentification SS_ID are, in this example, the following:

-   -   System ID (3 bits, value range 0 . . . 7) xxx-----    -   SV/Slot ID (6 bits, value range 0 . . . 63) ---xxxxxx

In other words, the three most significant bits indicate the satellitesystem and the last six bits indicate the satellite.

The specification for the system ID is disclosed in Table 2.

TABLE 2 System ID System ID value GPS 0 SBAS 1 Galileo 2 GLONASS 3 QZSS4 LAAS 5 Reserved for future use 6 Reserved for future use 7

SV/Slot ID is the satellite index in the broadcasted navigation model.

The second field of the second section contains Carrier Frequency Index.This parameter is a GLONASS specific frequency channel index (the mapbetween the satellite index indicating a slot in the orbit and thenavigation signal frequency. This map is included in the GLONASS almanacbroadcast). It is set to 0 for any other system than GLONASS. Valuerange for this field is [−7, −13].

The third field of the second section contains Fit Interval. This fieldspecifies the validity period of the navigation model. Value range forthis field is 0.125-448 h. This parameter is specified according to aspecial floating-point representation as described in the Table 3 below.

TABLE 3 Exponent, e Mantissa, m Floating-Point Fit Interval (3 bits) (3bits) value, x Value, F 0 0 0.125 F < 0.125 h 0 1 0.25 F = 0.25 h 0 1 <m < 8 (m + 1) * 2⁻³  F = {0.375 h, 0.5 h, 0.625 h, 0.75 h, 0.875 h, 1.0h} 1 < e < 7 0 <= m < 8 (m + 1) * 2^((e−1)) F = x h 7 0 <= m < 7(m + 1) * 2^((e−1)) F = x h 7 7 512 F = Infinite

The Fit Interval value of 63 (=2⁶−1) has a special meaning that definesinfinite interval for the Navigation Model of the specific satellite.

The fourth field of the second section contains SV Health. Thisparameter gives information about the satellite's current health. Thehealth values are GNSS system specific (see for instance ICD-GPS-200).

The fifth field of the second section contains Issue Of Data. Issue ofData field contains the identity for the Navigation Model. In the caseof, for instance, broadcasted GPS ephemeris, the 10 least significantbits (LSB) of IOD contain the IODC index as described in GPS-ICD-200.The MSB of IOD is set if the Navigation Model is not based on anybroadcasted ephemeris, but is based on long-term fit that is providedfrom a source external to the navigation systems.

The third section relates to the Satellite Clock Model. The first fieldof the third section contains t_(oc), which informs least significantbits of the reference time for the clock model. The 12 MSBs are includedin the first section in the field t_(oe) _(—) MSB. The second a_(f2),third a_(f1) and fourth field a_(f0) of the third section contain2^(nd), 1^(st) and 0^(th) order coefficients for the clock model.

The fifth field of the third section contains T_(GD), which indicatesequipment group delay between L1 and L2 broadcasts. This parameter isdefined for GPS and GLONASS systems.

The fourth section relates to the first mode, which is the SatelliteNavigation Model Using Keplerian Parameters.

The navigation model as defined by using the Keplerian parameters is thesame as defined for GPS in GPS-ICD-200. This set of parameters is usedin the mode 1, i.e. with assistance data relating to satellites of theGPS and Galileo systems. The explanation of the model parameters isgiven in the Table 4 below.

TABLE 4 Parameter Explanation t_(oe) Time of ephemeris See theexplanation for t_(oe)_MSB. e Eccentricity (A)^(1/2) Square-root ofsemi-major axis M₀ Mean anomaly Ω₀ Longitude of the ascending node i₀Inclination @ t_(oe) ω Argument of perigee Δn Mean motion correction{dot over (Ω)} Rate of change of longitude of the ascending node {dotover (i)} Rate of change of inclination C_(us) Sine correction oflatitude C_(uc) Cosine correction of latitude C_(rs) Sine correction ofradius C_(rc) Cosine correction of radius C_(is) Sine correction ofinclination C_(ic) Cosine correction of inclination

Keplerian parameters are the native format for GPS and Galileo. However,the native format for GLONASS and SBAS differs from the format used byGPS and Galileo. Although it is possible, by using history data onorbits, to convert GLONASS and SBAS format to GPS/Galileo-type orbitmodel, it is advantageous to allow for the native GLONASS and SBASbroadcasted orbit model format to be included in the generalized model.This is advantageous from the LAAS point-of-view as well, sincerepresenting a stationary object with Keplerian parameters would requireadding significant number of bits to parameters Δn and {dot over (Ω)},if the pseudolite were to be kept substantially stationary. Moreover,Keplerian parameters can only present the object position at an accuracyof few cm. However, with pseudolites it is essential to have a sub-cmresolution (i.e. resolution smaller than 1 cm) in order to be able toobtain the best possible navigation solution. The use of GLONASS/SBASnative format in the model allows for representing the position of aLAAS transmitter in plain ECEF-coordinates without additional formatconversions.

The fifth section relates to the second and third mode, which are theSatellite Navigation Models Using ECEF Coordinates.

This set of parameters is used in the mode 2 (i.e. for LAAS) and mode 3(i.e. for GLONASS and SBAS).

TABLE 5 Parameter Explanation Modes t_(oe) Time of ephemeris 2, 3 Seethe explanation for t_(oe)_MSB. X MSB MSB of the x-coordinate in theECEF frame 2, 3 Y MSB MSB of the y-coordinate in the ECEF frame 2, 3 ZMSB MSB of the z-coordinate in the ECEF frame 2, 3 X LSB LSB of thex-coordinate in the ECEF frame 2, 3 Y LSB LSB of the y-coordinate in theECEF frame 2, 3 Z LSB LSB of the z-coordinate in the ECEF frame 2, 3 X′x-velocity in the ECEF frame 3 Y′ y-velocity in the ECEF frame 3 Z′z-velocity in the ECEF frame 3 X″ x-acceleration in the ECEF frame 3 Y″y-acceleration in the ECEF frame 3 Z″ z-acceleration in the ECEF frame 3

The number of most significant bits (MSB) in the position field ischosen so that GLONASS and SBAS range requirements are met. However, thenumber of MSB is also sufficient to represent QZSS orbits if required.

The number of LSB, on the other hand, represents the resolutionrequirement set by LAAS (resolution 3.9 millimeters).

The sixth section relates to the Satellite Position Accuracy Model. Itcontains two fields. The first field contains the parameter r₀, whilethe second field contains the parameter r₁. These parameters can be usedto describe the navigation model error propagation in time byError(t)=r₀+r₁·(t−t_(REFERENCE)).

For GPS the parameter r₀ is the URA (User Range Accuracy) value asdescribed in GPS-ICD-200 specification.

When it is necessary to transmit the navigation system assistance datamessage in the communications network, e.g. from the network element Mto the device R, the information is mapped into one or more messagesapplicable in the communications network. For example, in GSMcommunications network there is a certain message delivery approach(Radio Resource Location Services Protocol, RRLP) formed fortransmission of location related information. This approach is definedin the standard 3GPP TS 44.031, which defines the format of the assistedGPS data exchanged between the network element M and the device R. Inthis invention, this approach can be used to send the more generalhealth data to the device R.

In the network element M the available navigation information such asDGPS correction, ephemeris and clock correction and almanac data ismapped into corresponding fields of the assistance data message(s). Theephemeris, clock correction, almanac and other data relating to aparticular satellite are obtained from a satellite navigation message ofthat satellite or from an external service X. The message is received bythe reference receiver C or by a reference receiver in the externalservice module X. The assistance data message comprises a Cipher Controlelement to indicate if the information is ciphered or not, CipheringSerial Number element, and Data Information Element. The DataInformation Element (Data IE) carries the navigation information. Theelements are depicted in Table 6 below.

The Assistance Data message is built so that it is fitted into a fixedlength message not necessary occupying the whole message. It can containthree data sets: DGPS correction, ephemeris and clock correction,almanac and other data information. In case that the fixed lengthmessage has less information elements than bits available then the restof the message is filled with fill bits. Undefined spare bits areusually not allowed between elements. In an example embodiment thechannel to broadcast the Assistance Data message is e.g. CBCH (ControlBroadcast Channel) over which the SMSCB DRX (Short Message Service CellBroadcast, Discontinuous Reception) service is used. One SMSCB messagehas fixed information data length of 82 octets and the maximum length ofGPS Assistance Data is 82 octets. The device R can identify the LCSSMSCB message with Message Identifiers declared in 3GPP TS 23.041.

TABLE 6 Occur- Pres- Parameter Bits Resol. Range Units rences enceCipher Cipher 1 — 0-1 — 1 M Control On/Off Ciphering 1 — 0-1 — 1 M KeyFlag Ciphering Serial 16 — 0-65535 — 1 C Number Data 638 — — — — M

In FIG. 5 an example assistance message A according to an exampleembodiment of the present invention is shown. The message comprises thet_(oe) _(—) MSB, i.e. the 12 most significant bits (MSBs) of the time ofephemeris t_(oe) and the reference time for the clock model t_(oc) givenin UTC Time. That parameter is followed by a number of assistance datarecords A.2 (ADATA1, ADATA2, . . . , ADATAn). Each assistance datarecord A.2 contains assistance data relating to one satellite S1, S2 ofa navigation system. As a non-limiting example, the first and seconddata record of the message A could contain assistance information of twosatellites of the GPS system and the third data record of the message Acould contain assistance information of one satellite of the Galileosystem.

The structure of the assistance data record A.2 is depicted below themessage A in FIG. 5. The assistance data record A.2 comprises e.g. theSatellite and Format Identification record A.2.1, the Clock Model recordA.2.2, the Navigation Model record A.2.3, and the Position AccuracyModel record A.2.4. It is also possible to define more or less recordsfor the assistance data record A.2 than these four different recordsA.2.1, . . . , A.2.4.

The structure of the Navigation Model record A.2.3 is also depicted inFIG. 5 and it contains the fields of the third section of Table 1 asdisclosed above in more detail. For example, if the assistance data ofthe Navigation Model record A.2.3 contained data on GPS, Galileo or QZSSsystem, the structure indicated by Mode 1 in FIG. 5 could be used.Respectively, if the assistance data of the Navigation Model recordA.2.3 contained data on LAAS system, the structure indicated by Mode 2in FIG. 5 could be used, and, if the assistance data of the NavigationModel record A.2.3 contained data on GLONASS or SBAS system, thestructure indicated by Mode 3 in FIG. 5 could be used. It may also bepossible to use e.g. the structure of Mode 3 with satellites of the QZSSsystem.

It should be noted here that each Navigation Model record A.2.3 of theassistance message A should contain all the fields of the respectivemode. The mode selection can be based on the navigation system theparameters relate to, or another selection criteria can be used toselect the mode for transmission of the assistance data, wherein theselected mode is not necessarily dependent on the navigation system.

Now, an example situation on the usage of the assistance message formataccording to the present invention will be described in the following.The network element has storage area M.4.1 in the memory M.4 for storingnavigation data received from the reference receiver C. If there is nonavigation data stored e.g. of the satellites of the first navigationsystem, the controller M.1 of the network element forms a query message(not shown) and transfers it to the first communication block M.2 of thenetwork element. The transmitter M.2.1 makes protocol conversions, ifnecessary, to the message and transmits the message to the referencereceiver C of the first navigation system. The receiver C.3.2 of thesecond communication block of the first reference receiver C receivesthe message, makes protocol conversions, if necessary, and transfers themessage to the controller C.1 of the reference receiver C. Thecontroller C.1 examines the message and determines that it is a requestto transmit navigation data to the network element M. If the memory C.4contains the requested navigation data, it can be transmitted to thenetwork element M, unless there is a need to update the navigation databefore the transmission.

After the navigation data is updated, the controller C.1 of thereference receiver forms a message containing the navigation data andtransfers it to the transmitter C.3.1 of the second communication blockof the first reference receiver C. The transmitter C.3.1 transmits,after protocol conversions if necessary, the navigation data to thenetwork element M. The receiver M.2.2 of the network element receivesthe message, makes protocol conversions, if necessary, and transfers themessage to the controller M.1 of the network element, or stores thenavigation data received in the message directly to the memory M.4 ofthe network element. The memory may comprise certain areas (M.4.1, M.4.2in FIG. 3) for storing navigation data of satellites of differentnavigation systems. Hence, the data is stored to the area which isreserved for the navigation system from which the navigation data wasreceived.

The assistance data can be transmitted to the device R for example byrequest or by a broadcast transmission, e.g. on a control channel of thecommunications network P. In the GSM system a GPS Assistance DataBroadcast Message format is defined which can be used in such broadcasttransmissions for GPS. The assistance data is included in the messageutilising the format defined in this invention. For example, thecontroller M.1 of the network element M examines which kind ofnavigation data there is stored in the memory M.4. If, for example, thememory comprises navigation data of one or more satellites of the firstnavigation system and navigation data of one or more satellites of thesecond navigation system, the controller M.1 can construct theassistance message A in the assistance data message storage area M.4.3in the memory M.4 e.g. in the following way. The controller M.1retrieves the time of ephemeris t_(oe) from the navigation data andstores the 12 most significant bits of the time of ephemeris into firstfield A.1 of the message A.

It should be noted here that the definition of time in this assistancedata format is different from the present GPS time. As mentionedearlier, for instance, GPS time rolls over every week. The new timedefinition does not do this. Moreover, the manner in which time isdefined is not relevant from the point of view of the invention.

Then, the controller browses the navigation data of the first navigationsystem stored in the first storage area M.4.1 to form the firstassistance data record A.2 (ADATA1). The controller M.1 determines(M.1.2) the type of the system and sets (M.1.1) the first three bits ofthe SS_ID field in the Satellite and Format Identification record A.2.1accordingly. The other six bits are set on the basis of the number ofsatellite the navigation data of which is in question. In acorresponding way the other fields of the Satellite and FormatIdentification record A.2.1 are filled. Also the fields of the ClockModel record A.2.2 are filled on the basis of the reference time and thecoefficients of the clock model. The equipment group delay T_(GD)between L1 and L2 broadcasts is filled if the assistance data relates toa satellite of the GPS or GLONASS system. The parameter T_(GD) may beneeded in other systems as well.

The usage of the Navigation Model records A.2.3 depends on thenavigation system i.e. the controller M.1 selects one of the availablemodes Mode 1, Mode 2, Mode 3 or some other additional mode not mentionedhere.

The Position Accuracy Model record A.2.4 is also filled to inform thenavigation model error propagation in time.

If there is navigation data of another satellite of the first navigationsystem in the memory M.4, the controller M.1 of the network elementforms the second assistance data record A.2 (ADATA2) accordingly.

When assistance data records A.2 are formed from the navigation datastored in all the navigation data storage areas M.4.1, M.4.2, theassistance data message can be transmitted to the communicationsnetwork. The controller M.1 transfers the data in the assistance datamessage storage area M.4.3 to the second communication block M.3 of thenetwork element. The transmitter M.3.1 of the second communication blockof the network element M performs the necessary operations for formingthe signals for transmission carrying the assistance data, and transmitsthe signals to the communications network P.

The signals are received by the receiver R.2.2 of the communicationblock of the device R. The receiver R.2.2 demodulates the data from thereceived signals and e.g. transfers the data to the controller R.1 ofthe device R. The controller R.1 stores the data into the memory R.4 ofthe device R and examines (R.1.1) the assistance data. The examinationcomprises determining the mode of each received assistance data record.The examination may also comprise determining (R.1.2) the navigationsystem. Indication on the mode can be transferred to the positioningreceiver R.3 e.g. through the output line R.1.3 of the controller R.1.However, it is also possible that the controller R.1 is also used in thepositioning operations wherein it may not be necessary to transfer thedata (the mode and the assistance data) to the positioning receiver R.3but the controller R.1 can use the data stored in the memory R.4.

The memory R.4 can comprise a storage area R.4.1 for storing thenavigation data received in the assistance data messages. Navigationdata can also be received, in some circumstances, from satellites bydemodulating received satellite signals.

When the assistance data is retrieved from the assistance datarecord(s), they can be kept in the memory and used in the positioning.For example, when the positioning receiver R.3 can only demodulatesignals from one or two satellites, the positioning receiver R.3 can usethe assistance data for performing the positioning as is known as such.

The device R can perform the positioning at certain intervals, or when apredetermined condition is fulfilled. The predetermined condition caninclude, for example, one or more of the following situations: the userinitiates to a call e.g. to an emergency centre; the user selects apositioning operation from a menu of the device R; the device R and thecommunications network P perform a handover to another cell of thecommunications network P; the communications network P sends apositioning request to the device R; etc.

It is also possible that the communications network, e.g. the networkelement M requests the device R to perform positioning. The request canbe sent using the RRLP message delivery mechanism. Also the reply can besent using the RRLP message delivery mechanism.

When the positioning is to be performed, the positioning receiver R.3 orthe controller R.1 of the device can examine whether there is enoughup-to-date navigation data stored in the memory R.4. If some navigationdata is not up-to-date (i.e. the navigation data has become older than apreset time), or some necessary navigation data is missing, the devicecan form and send a request message to the communications network P, forexample to the base station B, which forwards the request message to thenetwork element M. The network element M gathers the requestednavigation data and forms a reply message. The reply message is thentransmitted via the serving base station B to the device R. The receiverR.2.2 of the communication block R.2 of the device receives anddemodulates the reply message to retrieve the navigation data. Thenavigation data is stored e.g. into the navigation data storage areaR.4.1 of the memory R.4.

In another embodiment of the present invention the network element Mperforms at least some of the positioning calculation in this embodimentthe device R assists the network element M by performing e.g.carrier-phase measurements and transmitting the measurement results tothe network element M in a measurement information message (GNSSmeasurement information). The network element M also forms assistancedata by receiving navigation data from a reference receiver C or thenetwork element M receives the assistance data from the assistance dataserver X. Then, the network element M calculates the position of thedevice R by using the measurement data and assistance data. Anotheroption is that the position calculation is performed in another server(not shown) wherein the network server M transmits the measurementresults and the assistance data to the another server.

In a yet another embodiment the device R performs pseudorangemeasurements and transmits the measurement results to the networkelement M in a measurement information message (GNSS measurementinformation). The network element M uses the measurement results andassistance data formed by the network element M or received from theassistance data server X. Then, the network element M calculates theposition of the device R by using the pseudorange measurement data andassistance data, or the network element M transmits the pseudorangemeasurement data and assistance data to another server (not shown),which performs the position calculations.

In these above mentioned embodiments the measurement informationtransmitted from the device R to the network element M may depend on thenavigation system but still the principles presented above can be usedfor forming a general message, which is independent on the navigationsystem.

The core of the invention is in the multi-mode functionality. Thesatellite system (GPS, Galileo, GLONASS, SBAS, LAAS, QZSS, or someother) that is indicated by the MSBs of the SS index, may define themode. However, the mode may also be decided by using other factors. Themode then defines the orbit model mode and in certain implementationsalso the clock model mode.

Clearly, the method of indexing satellites (i.e. the navigation modelidentification contains information on the system and the SV) is anessential element in the invention. For GLONASS, Carrier Frequency Indexis vital (in addition to the SS index).

It is noteworthy that the clock model is common for all the modes (andtherefore for all the navigation systems) in this exampleimplementation. However, the clock model may also change with the mode.

It should be noted that the navigation assistance message specifiedcontains various items (specifically, t_(oe) _(—) MSB, fit interval, SVhealth, IOD, t_(oc), T_(GD), t_(oe), r₀, r₁) that are, of course,important for the navigation model to function properly, but are notimportant from the point-of-view of this invention (these parameters arein brackets in the table defining the format). For instance, thereference time for the model can be given in various ways (now, t_(oe)_(—) MSB, t_(oc) and t_(oe)), but changing it does not affect themulti-mode functionality. As another example, the fit interval isdefined as a floating point value (Table 3 above). This is just anexample and the fit interval can also be specified in some other meanstaking system specific issues into account. The parameters, which arenot important from the point-of-view of the current invention, are onlygiven for the sake of completeness.

Also, it should be emphasized that the actual bit counts and scalefactors are subject to change, if new specifications or clarificationsshould appear. Changing the bit counts and/or scale factors does notchange the spirit of the invention. For instance, adding resolution tovelocity components would not be a different invention. As a yet anotherexample, consider the SS ID. The indexing method currently used instandards is able to differentiate only between GPS satellites. The nowproposed SS ID contains information on the system and the satellite.These two can be expressed in the same field, but it is not necessary todo so (given that the system is defined in some other field). Hence, asimple modification of the fields would not, again, change the spirit ofthe invention.

The communications network P can be a wireless network, a wired network,or a combination of these. Some non-limiting examples of thecommunications networks have already been mentioned above but WLAN andWiMax networks can also be mentioned here.

The operations of the different elements of the system can mostly becarried out by software, i.e. the controllers of the elements operate onthe basis of computer instructions. It is, of course, possible that someoperations or parts of them can be “hard coded” i.e. implemented byhardware.

The invention claimed is:
 1. A network element comprising a controllingelement for forming assistance data of a generalized navigation modelincluding at least one clock model and at least one orbit model to beused to characterize satellite clock behaviour and satellite orbit inmore than one navigation system, wherein said assistance data containsan indication of the navigation system to which said assistance dataapplies; and a transmitting element for transmitting the assistance datato a communications network; and further wherein the controlling elementselects a format of the assistance data for the transmission of theassistance data; and constructs the assistance data according to theselected format.
 2. The network element according to claim 1, whereinthe network element further comprises a memory for storing navigationdata of the at least one satellite navigation system; and an examiningelement adapted to examine the navigation data to determine thenavigation system the navigation data relates to.
 3. The network elementaccording to claim 2, wherein the controlling element is adapted to formsaid assistance data on the basis of the navigation data.
 4. The networkelement according to claim 2, wherein the network element also comprisesa receiver for receiving navigation data of the at least one satellitenavigation system.
 5. The network element according to claim 2, whereinsaid navigation data also comprises an indication on the satellite thenavigation data relates to, wherein the determining element is alsoadapted to insert indication on the satellite into the assistance data.6. The network element according to claim 1, wherein said assistancedata comprises one or more assistance data records.
 7. The networkelement according to claim 1, wherein the assistance data record has atleast one of the following formats: a Keplerian model; Position inEarth-Centered Earth Fixed-coordinates; or Position, velocity andacceleration in Earth-Centered Earth Fixed-coordinates.
 8. The networkelement according to claim 1, wherein the communications network is acellular network.
 9. The network element according to claim 1, whereinthe network element is a mobile switching centre of a GSM system. 10.The network element according to claim 1, wherein said assistance datarelates to at least one of the following: the Global Positioning System;the GLONASS; the Galileo; the Quasi-Zenith Satellite System; a SpaceBased Augmentation System; or a Local Area Augmentation System.
 11. Thenetwork element according to claim 1, wherein the controlling element isadapted to select the format of the assistance data on the basis of thenavigation system the assistance data relates to, wherein saidindication of the navigation data also indicates the selected format ofthe assistance data.